1. Field of the Invention
The present invention relates to a ground detection apparatus for detecting the ground of a high-voltage DC power supply mounted on an electric vehicle.
2. Description of the Related Art
In an electric vehicle such as an electric automobile or a hybrid electric automobile, in general, a high-voltage DC current connected to a high-voltage battery is electrically insulated from a vehicle electrical circuit connected to a body of vehicle. For this reason, a ground detection apparatus is arranged for generating a warning when dielectric breakdown occurs between the high-voltage circuit and the body to decrease an insulation resistance to generate ground.
An example of a ground detection apparatus for conventional electric vehicle will be described below. In FIG. 1, reference numeral 110 denotes a traveling drive circuit system, reference numeral 51 denotes a battery group arranged as a high-voltage DC power supply (e.g., 200 to 300 V) and electrically insulated from a body B.
Reference numeral 52 denotes an inverter as a DC-AC converter, reference numeral 53 denotes a three-phase AC motor for vehicle traveling, reference numeral 54 denotes a plus bus serving as a DC positive electrode feeder extending from the battery group 51 to the inverter 52, reference numeral 55 denotes a minus bus serving as a DC negative electrode feeder extending from the battery group 51 to the inverter 52, and reference numerals 56, 57, and 58 denote an U-phase line, a V-phase line, and a W-phase line which are AC feeders extending from the inverter 52 to the AC motor 53, respectively.
A conventional ground detection apparatus 100 shown in FIG. 1 is to detect the ground to the body B in the traveling drive circuit system 110, and is constituted by an oscillation circuit 60 and a detection unit 80 for detecting a change in voltage level.
A connection point P between the oscillation circuit 60 and the detection unit 80 is connected to the plus bus 54 of the battery group 51 of the traveling drive circuit system 110 through a coupling capacitor 70A, and a DC component is isolated.
The oscillation circuit 60 comprises an oscillator 61 in which a multi-vibrator is formed by an operational amplifier or the like to generate a square wave having a predetermined frequency, an impedance converter/buffer 62 which is arranged to prevent the oscillation frequency of the oscillator 61 from being changed when a load impedance changes in generation of ground in the traveling drive circuit system 110, and a detection resistor 63 connected between the output stage of the impedance converter 62 and the coupling capacitor 70A. Reference numerals 65 and 66 denote protecting diodes for protecting the impedance converter 62 from a backward voltage or an overvoltage in generation of ground.
In the detection unit 80, a comparator 81 for comparing a voltage level of the connection point P between the detection resistor 63 and the coupling capacitor 70A at which an AC signal output of the oscillation circuit 60 appears with a reference voltage. The connection point P is connected to an inverted input terminal. To a non-inverted input terminal of the comparator 81, a reference voltage circuit is connected, where the reference voltage thereof is set by partial-voltage resistors 88 and 89.
A smoothing circuit 86 in which a time constant is set by a resistor 84 and a capacitor 85 is arranged at the output terminal of the comparator 81. An output from the comparator 81 is inputted to a non-inverted input terminal of a comparator 87 of the output stage through the resistor 84 of the smoothing circuit 86.
The time constant of the smoothing circuit 86 is set such that a smoothing voltage is lower than a reference voltage when an output from the comparator 81 is a duty ratio of 50%, and the smoothing voltage is higher than the reference voltage when an output from the comparator 81 is a duty ratio of 100%.
A reference voltage circuit for setting a reference voltage set by partial-voltage resistors 93 and 94 depending on the smoothing voltage of the smoothing circuit 86 is connected to the inverted input terminal of the comparator 87.
In the ground detection apparatus 100, reference numerals 91 and 92 denotes protecting diodes for protecting the comparator 81 from a backward voltage or an overvoltage in generation of ground.